History of the Prior Art
There are many production formations encountered in the drilling of modern oil wells wherein it is desirable to effect the perforating of the casing and the adjoining production formation in an "underbalanced" condition, i.e., where the fluid pressure within the casing immediately adjacent the production formation, and the tubular string connected to such casing region, are maintained at a substantially lower fluid pressure than the anticipated fluid pressure of the production formation. When the perforating gun is fired under these conditions, the production fluid flows rapidly through the resulting perforations into the casing and up the tubular string, carrying with it all debris resulting from the perforating operation, hence cleaning the fractures in the perforated formation for more efficient production flow when the well is completed. It is also sometimes desirable to minimize debris settlement from kill fluids or mud systems on top of go-devil actuated firing heads. A gun pressure activated sliding sleeve allows the operator to maintain clean fluids above the firing head up until the time the well is perforated. Upon perforation, the sleeve opens and allows well fluids to flow up the work string.
In order to efficiently utilize the underbalanced perforating, a packer is generally run into the well on a tubular string and set immediately above the region of the well casing to be perforated. The perforating gun is suspended from the packer by a tubular string which is fluid connected to a tubular work string. Other tools, such as chemical treatment, washing and/or gravel packing tools, may be connected in the same tool string either above or below the packer. In any event, the assembled tool string is run into the well with an essentially dry condition existing in the tool string, thus assuring that the fluid pressure in the tool string is substantially below that anticipated to exist in the formation to be perforated.
To permit the free flow of production fluid from the perforated formation, it is necessary that radial ports be opened in the tubular string below the packer and above the perforating gun. It has previously been proposed, for example, in U.S. Pat. No. 4,151,880 (Vann), that a radially ported, wire line actuated sleeve valve be mounted in this position. The utilization of a wire line actuated valve necessarily introduces delay in effecting the opening of the radial ports in the tubing string, thus delaying the initial flow of production fluid from the perforation formation. U.S. Pat. No. 4,299,287 (Vann) proposes to use a freely falling detonating bar to shift the sleeve valve. This can result in slowing the speed of the detonating bar to a level insufficient to fire the gun.